Superconductor covered with reinforced aluminum matrix

ABSTRACT

Disclosed herein is a superconductor covered with a reinforced aluminum matrix, which is obtained by cladding a superconductor with an aluminum matrix reinforced with a very small proportion of alumina dispersed therein and subjecting the clad superconductor to integral forming by codrawing.

BACKGROUND OF THE INVENTION

This invention relates to a superconductor covered with a reinforcedaluminum matrix and to a method for the manufacture thereof. Moreparticularly, the present invention relates to a superconductor coveredwith an aluminum matrix which makes the most of the high electricconductivity, the high thermal conductivity and the magneto resistanceeffect saturation characteristic inherently exhibited by high-purityaluminum at very low temperatures in an extremely high magnetic fieldand tensile strength and enjoys a notably improved tensile strength suchas has heretofore been found difficult to attain by the prior art and toa method for the manufacture of said aluminum superconductor.

As is universally known, a superconductor has a thermal conductivity onthe order of 10⁻³ to 10⁻² W/cm.deg. which is so low that when anelectric current of high current density flows superconductively throughsaid superconductor, the superconductor generates heat due tomagnetization, magnetic flux jump or some other cause. Consequently thesuperconductor transiently or partially shifts to a normally conductivestate. Because of this phenomenon, there has been suggested a method forstabilizing the behavior of the superconductor by cladding saidsuperconductor with a normally conductive metal possessed of highelectric conductivity and high thermal conductivity. To date, copper ofhigh purity has been used preponderantly as the normal conductive metal.Copper, however, has a disadvantage that the electric resistance thereofincreases notably in proportion the magnetic field.

In contrast, aluminum of high purity has its electric resistanceincreased in a high magnetic field to only about twice the value in anabsence of magnetic field. It enjoys the saturation characteristic thatthe electric resistance no longer varies even if the magnetic field isintensified further. Moreover, aluminum excels copper in electricconductivity and thermal conductivity and has light weight. Thus,aluminum possesses outstanding characteristics as the normal conductivemetal. When aluminum of high purity is deposited to directly cladsuperconductors such as, for example, of an Nb-78% Ti wire and the cladwires are subjected to wire-drawing, because of a broad difference inhardness between the two metals, there ensues a flowing phenomenonwherein only the aluminum layer on the surface is drawn out. Thus, thedesired integral forming of the two metals cannot be obtained byco-drawing. Formerly, the present inventors proposed an aluminum cladmultiplex superconductor having a uniform cross-sectional distributionof components throughout the entire length by precluding the flowingphenomenon which would otherwise occur while an aluminum clad multiplexsuperconductor was undergoing wire-drawing (U.S. Pat. Nos. 3,714,371 and3,778,895). According to the invention of said U.S. patents, thealuminum clad multiplex superconductor is obtained by placing at leastthree twisted or braided superconductive wires inside a pipe of highlypure aluminum, sheathing said aluminum pipe with a pipe of an aluminumalloy having a Vickers' hardness of not less than 50 and subjecting theresultant composite cable to the wire-drawing, whereby thesuperconductive wires and the aluminum pipe are integrally drawn out andbrought into intimate mutual cohesion. The wire-drawing to be performedon the aluminum pipe and the superconductive wires during theaforementioned manufacture of the aluminum clad superconductor, however,necessitates highly advanced skill. Further, the manufacture of saidaluminum clad superconductor necessitates use of at least three twistedsuperconductive wires.

The object of this invention is to provide a superconductor covered witha reinforced aluminum matrix which makes the most of the high electricconductivity, the high thermal conductivity and the magneto resistanceeffect saturation characteristic inherently exhibited by aluminum ofhigh purity at very low temperature and enjoys a notably improvedtensile strength and also a method for the manufacture of said aluminumclad superconductivity of high tensile strength.

SUMMARY OF THE INVENTION

To accomplish the object described above according to the presentinvention, there is provided a method for the manufacture of asuperconductor covered with a reinforced aluminum matrix, which methodcomprises cladding superconductor with an aluminum matrix having aluminadispersed therein.

The dispersion type reinforced alloy such as is obtained by havingalumina suitably dispersed in a matrix of highly pure aluminum asdescribed above retains, substantially intact, the advantageouscharacteristics of high electric conductivity and high thermalconductivity exhibited by aluminum of high purity at very lowtemperature and acquires the high tensile strength as well as themagneto resistance effect saturation characteristic inherently possessedby aluminum. When this reinforced aluminum is used as the material forstabilizing the superconductor, it enjoys ease of fabrication equal toor even superior to that of the conventional material of copper. When amultiplicity of core wires are embedded in the matrix and the resultantcomposite is subjected simultaneously to the wire-drawing, integralforming of cable can easily be carried out without entailing the flowingphenomenon.

The term "aluminum" as used in the present specification refers toaluminum of high purity sufficient for the residual resistivity at verylow temperatures to fall below 5×10⁻⁸ Ω·cm.

The other objects and characteristics of the present invention willbecome apparent from the description to be given in further detailherein below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing the relation between the magnetic field andthe resistivity ratio as determined of copper and aluminum.

FIG. 2 is a graph showing the relation between the resistivity ratio andthe weight ratio with reference to the equivalent magnet as formed byintegrally forming the superconductor with copper or aluminum.

FIG. 3(a) and (3b) are cross-sectional views illustrating thefundamental structure of the aluminum superconductor of high tensilestrength manufactured by the present invention.

FIGS. 4(a), 4(b), 4(c), 4(d), 4(e), 4(f), and 4(g) are cross-sectionalviews illustrating the process of manufacture of the aluminumsuperconductor of high tensile strength according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A superconductor, as is universally known, has a thermal conductivity onthe order of 10⁻³ to 10⁻² W/cm.deg. which is so low that thesuperconduction of electric current of high current density transientlyor partially shifts to normal conduction owing to generation of heatcausable such as by magnetization, magnetic flux jump or some othersimilar phenomenon. It has been well known to embed in thesuperconductor a normally conductive metal having high electricconductivity and thermal conductivity and thereby enable thesuperconductor to retain its stability even in such a difficultsituation as described above.

As the normally conductive metal, there has been particularly usedcopper of high purity. As indicated in FIG. 1, however, copper manifeststo an unusual extent the so-called magneto resistance effect, i.e. thephenomenon that the electric resistance increases conspicuously as theintensity of magnetic field increases. When copper is considered as thematrix for a superconductive electromagnet, for example, the resistivityincreases roughly by one decimal place, occasionally more, as themagnetic field gains in its intensity, although the magnet possesses adesired level of electric conductivity in the absence of mangetic field.This means that the copper content in the matrix must be increased inproportion to the intensity of the magnetic field to be generated,making the practical design of the electromagnet extremely difficult.What makes the matter worse is the fact that, as is evident from FIG. 1,the increase in the resistance due to the magneto resistance effectbecomes increasingly more conspicuous with the increasing purity ofcopper, so that in a high magnetic field, the advantageous electricaland thermal characteristics ascribable to the high purity of copper aresubstantially completely offset by the increasing resistance. An attemptto overcome this difficulty by making some adjustment or other on thecopper and consequently stabilizing the superconductive electromagnetfor generation of magnetic field necessitates use of an excessivelylarge amount of copper to ensure stable generation of high magneticfield. Consequently, the weight of the superconductive electromagnetwill become exceedingly large.

In contrast in the case of aluminum of high purity, the electricresistance is increased in an extremely high magnetic field to onlyabout twice the value in an absence of magnetic field, as is evidentfrom FIG. 1. Moreover, the aluminum manifests the saturationcharacteristic, a phenomenon that the electric resistance remains as itis despite any further increase in the intensity of magnetic field. Thisfact indicates that the electric resistance is hardly affected by thepurity of aluminum. When aluminum of high purity which enjoys theseadvantages is adopted as the matrix for stabilizing the magnetic field,there will be found numerous advantages such as simplicity of design,notable decrease in the amount of matrix required and generous reductionin the total weight of the superconductive electromagnet.

Now let d_(s) (kg/m³) stand for the density of the superconductive wire,d_(m) (kg/m³) for the density of the matrix, A/a=k for thecross-sectional area ratio between the matrix and the superconductivewire, ρ(Ωm) for the resistivity and l(m) for the length, and the weightm (kg) will be expressed as follows.

    m=(d.sub.s a+d.sub.m A)l                                   (1)

The electric resistance R (Ω) of the matrix will be expressed asfollows:

    R=ρl/A                                                 (2)

Substitution of the equation (2) in the equation (1) gives rise to thefollowing equation.

    m=ρl.sup.2 /P.sub.k (d.sub.s +kd.sub.m)                (3)

As the criterion for the comparison of two equivalent electromagnets,one using copper and the other aluminum on the basis of the theory ofstabilization of superconductor, the weight ratio M between copper andaluminum to be used as the stabilizing materials under the conditionshaving the values of R, l and ρ/K fixed will be expressed as follows(provided that there is used a superconductor having d_(s) =5.96×10³kg/m³ for Nb-Ti, d_(m) =2.7×10³ kg/m³ for Al and d_(m) =8.9×10³ kg/m³for Cu respectively): ##EQU1## In this case, the following equation isalways satisfied. Thus k(Cu) and k(Al) cannot assume their valuesindependently of each other.

FIG. 2 graphically represents the relation of formula (4) given above.From the graph, it is found that when there is selected a material whoseresistivity ratio (ρ(Cu)/ρ(Al) is 10, for example, k(Al)=1.0 is obtainedfor an electromagnet which is formed of aluminum matrix andsuperconductor at a ratio of 1:1 and that in an equivalent electromagnetusing a copper matrix instead of the aluminum matrix, k(Cu)=10 isobtained. In this case the weight ratio is 10, suggesting that theelectromagnet using the material according to the present invention hasa weight only one tenth of the weight of the countertype using copper asthe material. This relation does not reflect the magneto resistanceeffect. If said effect is taken into consideration, then there will beinevitably required due correspondence to a fairly large value of theratio of ρ(Cu)/ρ(Al). Consequently, the weight and the volume willbecome greater.

Though aluminum of high purity possesses more desirable properties thancopper, it has never been adopted for the stabilization of magneticfield. The reason is that since aluminum of high purity has low tensilestrength and unusually low hardness as already pointed out, thetechnical difficulty encountered in the course of its fabrication hasmade it impracticable to permit a multiplicity of core wires to besimultaneously embedded in the aluminum matrix or use the aluminum inhigh magnetic field without the presence of a reinforcing material.Aluminum of high purity, if used as the sole superconductor, exhibitsoutstanding properties. Under conditions involving heavy electriccurrent, large energy capacity and high magnetic field, however, thesuper-conductors of high-purity aluminum themselves suffer drawbacksfrom the standpoint of physical application. To be specific, as thesuperconductive electromagnet enlarges, the energy builds up therein tothe extent of exerting stress on various points of the electromagnet.Each of the stresses is divided into a radial compressive force and alinear tensile force. Since the requirements are generally stricter forthe linear tensile force than for the radial compressive force, thetensile strength alone will be discussed herein as the criterion forstrength for the sake of convenience. In the case of a solenoidelectromagnet, the distribution of stress with reference to the radialdirection from the center is expressed as follows:

    δ(r)=(10.sup.-6 /9.8) H(λ·J) r       (5)

Here, δ(r) denotes the linear stress (kg/cm²) at a distance of theradius (r cm) from the axis, H the magnetic field (gauss) and λ·J(A/cm²) the average current density in the cross section of the coil. Asis readily understood from this formula (5), where no reinforcingmaterial is used, the electric current permitted to flow through thesuperconductor is to be heavily restricted by the limited tensilestrength because the stress developing within the electromagnetincreases in direct proportion to the increase in the radius of theelectromagnet, notwithstanding the fact that the current density of thesuperconductor may successfully be increased by the stability which isbrought about by the use of aluminum of high purity.

The present invention has been accomplished with a view to overcomingthe disadvantage just described. To be more specific, the presentinvention has issued from successful realization of the attempt to makethe most of the excellent behavior exhibited by aluminum of high purityin the stabilization of superconductor and, at the same time, augment toa considerably higher extent the tensile strength and the hardness ofaluminum of high purity which are both low and has heretoforeconstituted the demerits thereof.

Now, the aluminum of high purity which is used in the present inventionwill be described. Normally, the individual particles of powderedaluminum of high purity have their surface covered with a very thinoxidized coat. Further, the individual particles of high-purity aluminumhave different specific surface areas (surface area/volume) from oneanother owing to the variation in their particle diameter. An experimentconducted by using two samples of aluminum powder, one having an averageparticle diameter of 37μ and the other 44μ respectively, for instance,will be described: Each sample of high-purity aluminum powder was placedto fill a rubber container, compacted into a unitary mass by a treatmentwith a static water pressure and finally given an integral forming.Thereafter, each sample was given a heat treatment at about 665° C. Thuswere obtained test specimen "A" (37μ of particle diameter) and testspecimen "B" (44μ of particle diameter). The two test specimens weremeasured for tensile strength. The results were as shown in Table 1below. The tensile strength of pure copper and that of pure aluminum areindicated in the table for comparison.

                  Table 1                                                         ______________________________________                                        Test Specimen A (37 μ of diameter)                                                             32 kg/mm.sup.2                                            Test Specimen B (44 μ of diameter)                                                             22 kg/mm.sup.2                                            Pure copper         about 23 kg/mm.sup.2                                      Pure aluminum       about  5 kg/mm.sup.2                                      ______________________________________                                    

It is seen from Table 1 that the test specimen "A" excelled pure copperin tensile strength. An aluminum composite having a tensile strengthequal to or superior to that of pure copper can be obtained by suitablyselecting the particle diameter of aluminum powder. In the individualaluminum particles, alumina is deposited to a thickness of about 30-80 Aon the surface of said particles. In consequence of said integralforming work, the alumina is finely divided and distributed throughoutthe mass of aluminum particles, giving desirable reinforcement to theresultant formed article of aluminum. The amount of alumina thusdispersed within the formed article is about 0.8% by volume in thealuminum powder 37μ in average particle diameter, about 0.3% by volumein the aluminum powder 44μ in average particle diameter and about 1.5%by volume in the aluminum powder 22μ in average particle diameterrespectively.

Then, three samples of aluminum powder having a fixed particle diameterof 37μ and differing in purity as indicated in the table below weresubjected to the rubber-compression treatment and the heat treatment asdescribed above. The resultant test specimens were measured for electricresistance at 4.2° K. The results of the measurement were as shownbelow.

                  Table 2                                                         ______________________________________                                        Purity of aluminum powder (%)                                                                   Electric resistance (Ω . cm)                          ______________________________________                                        99.9              ˜ 9 × 10.sup.-8                                 99.99             ˜ 2 × 10.sup.-8                                 99.999            ˜ 6 × 10.sup.-9                                 ______________________________________                                    

It is understood from the preceding table that all the aluminum powderstested were amply usable as the matrix in the superconductor.

Now, a description will be given concerning the superconductor whichuses as the cladding material the high-purity aluminum treated asdescribed above. FIG. 3(A) is a corss-sectional view of the fundamentalconstruction of a superconductor in accordance with this invention,comprising one superconductor 1 clad with a high-purity reinforcedaluminum matrix 2 of augmented tensile strength. FIG. 2(B) is a similarcross-sectional view, except a plurality of superconductors are cladwith the high-purity reinforced aluminum matrix. When thesuperconductor(s) is clad with a high-purity reinforced aluminum matrixof augmented tensile strength as illustrated therein, the excellentelectrical and thermal properties which aluminum of high purityexhibited as described above at very low temperatures are advantageouslyutilized in stabilizing the superconductor(s) and the augmented tensilestrength brought about by uniform dispersion of alumina particles isgreat enough to withstand various heavy stresses which build up withinthe superconductive electromagnet of a high magnetic field and a largeenergy capacity.

The method of the present invention for the manufacture of an aluminumclad superconductor will be specifically described below with referenceto FIG. 4.

A high-purity aluminum powder covered alumina of the nature ofspontaneous occurrence having a suitable particle diameter is placed tofill a rubber container, compacted into a unitary mass by a static waterpressure and heated at about 665° C. to produce an aluminum matrix 2'containing a multiplicity of bored holes 3.

Subsequently, a rod 4 of high-purity reinforced aluminum is held inposition as illustrated in FIG. 4(B) and a plurality high-purityaluminum pipes 5 are inserted therein. The rod 4 and the pipes 5 thuscombined are wholly inserted into a pipe 6 made of a material such as analuminum alloy which has a hardness equal to or greater than that ofhigh-purity aluminum. This pipe 6 has the sole function of reinforcingthe superconductor. It may be replaced with stainless steel wires,therefore, to suit the particular use to be involved. This insertion ofthe reinforcing material may otherwise be omitted completely asillustrated in FIG. 4(A). As the pipe 5 to be used for insertion asshown in FIG. 4(B), there is used that which has a sole superconductor 1clad with a high-purity reinforced aluminum matrix 2 of augmentedtensile strength as illustrated in FIG. 4(C) or that which has aplurality (three) superconductors 1 clad with a high-purity reinforcedaluminum matrix 2 of augmented tensile strength as shown in FIG. 4(D).In this case, the superconductor(s) may be inserted simply inside thepipe of high-purity aluminum of augmented tensile strength or thesuperconductor(s) stabilized with reinforced aluminum by integralforming as illustrated in FIG. 3 may be inserted. For the purpose ofprecluding possible loss of stability due to time-course change ofmagnetic field, the superconductor may be twisted with a suitable pitchand inserted in the twisted state into the aforementioned pipe. Thecomposite material obtained preparatory to the wire-drawing in the stateillustrated in FIG. 4(B) is now subjected to wiredrawing and thereafteris flattened by means of a roller, for example. The superconductor whichis thus finished in a flattened shape as illustrated in FIG. 4(E) hasthe advantage that the ratio of the volume occupied by thesuperconductive electromagnet to the entire volume of the superconductoris greater. FIG. 4(F) is a cross-sectional view of the constructionhaving a plurality of high-purity aluminum pipes 5 of augmented tensilestrength intertwined or braided with reinforcing wires 7 (such as ofstainless steel). Said high-purity aluminum pipes 5 of augmented tensilestrength have their outer surface 8 electrically insulated by anodicoxidation so as to alleviate possible loss caused by the combinedcurrent generated in consequence of the time-course change of magneticfield. This type of cable is adopted for applications involving a pulsecurrent or alternating current. FIG. 4(G), though depicting a conceptidentical with that of FIG. 4(F), illustrates a construction finished ina flattened shape by use of a roller, for example, with a view toincreasing the ratio of the volume occupied by the superconductiveelectromagnet to the whole volume involved. After such a fabrication, analumina layer having a powerful electrically insulating activity may beformed at any desired time by an anodic-oxidation coating treatment. Theelectric insulation of this type has thermal conductivity of a level twodecimal places higher than that of the conventional organic insulatingsubstance (such as, for example, polyvinyl formvar) and yet has athickness one decimal place smaller (a thickness less than 1μ beingsufficient). Consequently, it far excels the conventional insulatingsubstance particularly in heat-expelling effect. Further, thisinsulating coat withstands radioactive rays and neutral rays morestrongly than the organic insulating substances. Thus, the electricinsulation provided by this coat functions effectively against theradioactive rays such as from the nuclear fusion furnaces.

The reinforcement of the high-purity aluminum matrix in the presentinvention has been described as being effected through dispersion of thealumina layer formed on the surface of individual particles ofhigh-purity aluminum. The desired reinforcement may be accomplishedotherwise than by resorting to the method just described. For example,it may be accomplished by causing a suitable amount of alumina to bemixed into and dispersed throughout a molten mass of high-purityaluminum held in a crucible such as by means of ultrasonic waves.

As described above, the outstanding electrical and thermal propertiesexhibited by high-purity aluminum at very low temperatures can beutilized by the present invention for stabilizing the superconductor,while such utilization has heretofore been held impracticable. Besides,this invention permits the superconductor to acquire tensile strengthhigh enough to withstand various heavy stresses which build up in thesuperconductive electromagnet of high magnetic field and large energycapacity. Because of high current density, high ability to stabilize themagnetic field, light weight, etc., the superconductor to be obtained bythe present invention is believed to find a wide variety of uses, amongothers, as electromagnets for MHD power generation, electromagnets forparticle acceleration, electromagnets for nuclear fusion, electromagnetsfor lasers, electromagnets for pulse current generation, electromagnetsfor linear motor flotation, electromagnets for medical therapy, bendingmagnets for foam boxes, magnets for missiles and cables for powertransmission.

What is claimed is:
 1. A superconductor covered with an aluminum matrixof augmented tensile strength, which comprises at least onesuperconductive wire and an aluminum matrix laid to cover the outersurface of said at least one superconductive wire and having aluminadispersed therein, said at least one superconductive wire and saidaluminum matrix having been co-drawn.
 2. The superconductor according toclaim 1, wherein said aluminum matrix has alumina dispersed therein in atotal amount of from about 0.3 to about 1.5 percent by volume.
 3. Amultifilamentary superconductor comprising a plurality ofsuperconductive wires separated from one another and surrounded by amatrix of composite material consisting essentially of aluminum withalumina dispersed therein, the wires and matrix having been co-drawn.